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Forced vital capacity and forced expiratory volume in six seconds as predictors of reduced total lung capacity

Dept of¹ General Practice, and ² Respiratory Division, Academic Hospital, University of Brussels, Brussels, Belgium.

CORRESPONDENCE: J. Vandevoorde, University of Brussels (Vrije Universiteit Brussel), Dept of General Practice, Laarbeeklaan 103, B-1090 Brussels, Belgium. Fax: 32 24774301. E-mail: Jan.Vandevoorde{at}vub.ac.be

Keywords: Forced expiratory volume in six seconds, forced vital capacity, restrictive ventilatory defect, spirometry, total lung capacity

Received: March 16, 2007
Accepted September 19, 2007

The present study aims to derive guidelines that identify patients for whom spirometry can reliably predict a reduced total lung capacity (TLC). A total of 12,693 lung function tests were analysed on Caucasian subjects, aged 18–70 yrs.

Restriction was defined as a reduced TLC. Lower limits of normal (LLN) for TLC were obtained from the European Respiratory Society recommended reference equations. Reference equations from the National Health and Nutrition Examination Survey III were used for forced vital capacity (FVC) and forced expiratory volume in six seconds (FEV₆). The performance of FVC and FEV₆ to predict the presence of restriction was studied as follows: 1) using two-by-two (2x2) tables; and 2) by logistic regression analysis. Both analyses were performed in obstructive (defined as forced expiratory volume in one second (FEV₁)/FVC or FEV₁/FEV₆ <LLN) and nonobstructive subgroups, and separately for males and females.

The 2x2 tables showed generally low positive and high negative predictive values for FVC or FEV₆ below their LLN in predicting a reduced TLC. Logistic regression analysis showed that in nonobstructive subjects, restriction can be positively predicted if FVC or FEV₆ is <55% predicted (males) or <40% pred (females). Restriction can be ruled out if FVC or FEV in six seconds is >100% pred (males) or >85% pred (females).

In obstructive patients, spirometry cannot reliably diagnose a concomitant restrictive defect, but it can rule out restriction for patients with forced vital capacity or forced expiratory volume in six seconds >85% pred (males) or >70% pred (females).